Three reactors were in operation at Fukushima Daiichi plant when Friday's quake hit

(CNN) -- The crisis at an earthquake-damaged nuclear plant in Japan continued Tuesday, as radiation levels rose to dangerous levels following an explosion at a third overheating reactor.

Tokyo Electric Power Company, which manages the Fukushima Daiichi facility, said a fire at a fourth reactor on Tuesday morning may also have caused radioactive material to escape. Company officials said high temperatures inside the building housing the reactor may have caused spent fuel rods sitting in a pool to ignite or explode. The blaze was later extinguished.

Japan's Chief Cabinet Secretary Yukio Edano said radiation levels had spiked after the explosion and fire, but he later told reporters that readings at the plant's gate had returned to a level that would not cause "harm to human health."

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Officials had earlier reported radiation levels at the plant of between 100 and 400 millisieverts, or as much as 160 times higher than the average dose of radiation a typical person receives from natural sources in a year.

But Glenn Sjoden, a professor of Nuclear Engineering at Georgia Tech in the United States, told CNN that the radiation levels were extremely serious. "Things have dramatically changed in the last few days. Initially they were recording slightly elevated levels at the site boundary because they were venting small pumps from the buildings to release pressure.

"The situation is very different now as a fraction of the core has melted releasing some fission products to the atmosphere as they continue to relieve the pressure in the system."

However he said the radiation levels indicate that only a partial meltdown has occurred in the affected reactors. "The levels would be significantly higher if a total meltdown had occurred," he said.

A nuclear meltdown is where the fuel in the core of the reactor starts to melt. This happens when there is a loss of coolant (water) in the core. A fuel melt is a serious problem as it releases a lot of the radioactive material that is contained in that fuel.

What is being released into the atmosphere?

According to Sjoden, the main radioactive components likely to escape when a reactor's fuel rods are breached are Iodine-131 and Caesium-137. Damaging levels of both components were emitted during the Chernobyl reactor disaster in the Ukraine in 1986.

What are the effects on humans?

According to Malcolm Grimston, Associate Fellow for Energy, Environment and Development at London's Chatham House, the single biggest issue following the Chernobyl disaster was radioactive iodine getting into the thyroid gland and causing cancer.

However he said it was likely those living near the Fukushima plant will have been offered iodine tablets which flood the thyroid gland with non-radioactive iodine so there is no space for radioactive iodine to penetrate.

"At Chernobyl they had trouble getting hold of these tablets because it was such a poor area," he told CNN.

All but about 50 workers from the facility have been evacuated while people living within 18.6 miles (30 kilometers) of the plant have been advised to remain indoors.

Fuel rods are long metal tubes containing pellets of fissionable material which provide fuel for nuclear reactors. Once these fuel rods have been used, they are submerged in a reinforced steel and concrete container nearby filled with water -- not unlike a large swimming pool - to cool them and allow their radioactivity to decay.

As long as the spent fuel rods remain covered they are perfectly safe. If this changes then the likelihood of a problem developing increases.

"There's still long-lived radioactivity in the rods from the spent fuel," said Paddy Regan, Professor of Physics at Surrey University in England. This consists of a mixture of residues from the nuclear reaction that take a long time to decay.

"There are not many of them but they do take a lot of time to decay and they generate a lot of heat. That's why they're put in these cooling ponds."

What caused the fire in No.4 reactor?

This is not yet clear. High temperatures inside the building may have caused fuel rods sitting in the cooling pond to ignite, the plant's owner said.

According to Professor Andrew Sherry, Director of the Dalton Nuclear Institute at the University of Manchester, the water level may have dropped in the pool. "If that happens you don't have enough cooling water to reduce the heat, which is similar to what is happening in the reactor's core," he said.

"You haven't got criticality because the rods aren't packed together, but they are still hot. The remaining water could heat and then evaporate."

What about the possible damage caused by the blast in reactor No.2?

"My understanding is that the damage was associated with the torus structure underneath the reactor's containment vessel," said Sherry. This donut-shaped structure, also known as a wet well, helps to manage the pressure inside the reactor by condensing the steam into water.

"If there was a pressure drop at the time then this suggests it has been damaged and there may be a leak that led to more hydrogen and steam escaping in a less controlled manner," said Sherry. "That will have increased local radiation levels."

How did the crisis at the plant begin?

Three reactors were in operation when Friday's 9.0 magnitude quake struck. The reactors -- which went into service between 1970 and 1979 -- are designed to shut down automatically when a quake strikes, and emergency diesel generators began the task of pumping water around the reactors to cool them down. However, these stopped about an hour later. The failure of the back-up generators has been blamed on tsunami flooding by the International Atomic Energy Agency (IAEA).

What caused the explosions?

Grimston said a build-up of pressure inside the inner containment of the reactors was the likely cause. "My suspicion is that as the temperature inside the reactor was rising, some of the metal cans that surround the fuel may have burst and at high temperature, that fuel cladding can react with water to produce zirconium oxide and hydrogen.

"That hydrogen then will be part of the gases that need to be vented. That hydrogen then mixes with the surrounding air. Hydrogen and oxygen can then recombine explosively."